Conventional fiber optic cables comprise optical fibers that are used to transmit voice, video, and data information. Fiber optic cables may be required to meet mechanical and environmental tests, for example, as defined in Bellcore GR-409-Core, Issue 1, published May, 1994, and incorporated by reference herein. The mechanical tests of Bellcore GR-409-Core include, for example, tensile strength, compression resistance, cycle flex, and impact tests. In addition, the mechanical tests of Bellcore GR-409-Core include, for example, temperature cycling and cable aging. Fiber optic cables not able to withstand the rigors of the foregoing tests may be rejected by customers for certain applications. An example of a fiber optic cable that meets Bellcore GR-409-Core is disclosed in U.S. Pat. No. 5627932 assigned to the assignee hereof. In addition, fiber optic cables may be required to meet Bellcore GR-20-Core, which sets forth water penetration standards for optical cables intended for outdoor applications.
Indoor fiber optic cables have been developed for installation in plenums and risers, and/or ducts of buildings. In order for a fiber optic cable to be rated for riser or plenum use, the cable must meet flame retardance standards as determined by means of vertical or horizontal flame tests. Exemplary requirements for such tests have been established by Underwriters Laboratories (UL). Since riser cables are typically installed in vertical shafts, the relevant standard for riser rated fiber optic cables is embodied in UL 1666, a flame test in a vertical shaft without a forced air draft in the shaft. UL 1666 does not include a smoke evolution requirement. UL has promulgated the riser rating requirements in a document entitled "Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shafts", wherein values for flame propagation height are set forth. Examples of riser rated fiber optic cables are disclosed in U.S. Pat. No. 5748823 and EP-A1-0410621.
The relevant standard for plenum rated fiber optic cables is embodied in UL 910, a horizontal flame test setting forth flame propagation and smoke evolution requirements. In the construction of many buildings, a plenum can include, for example, a space between a drop ceiling and a structural floor above the drop ceiling. A plenum typically serves as a conduit for forced air in an air handling system, and the plenum is oftentimes a convenient location for the installation of fiber optic cables. If, in the event of a fire, the fire reaches the plenum area, flames that would otherwise rapidly propagate along non-plenum rated cables are retarded by plenum rated cables. Moreover, plenum rated cables are designed to evolve limited amounts of smoke. Riser rated cables tested to UL 1666 specifications typically do not exhibit acceptable flame spread and smoke evolution results and may be therefore unsuitable for plenum use.
The UL 910 test is promulgated by UL in a document entitled: "Test for Flame Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air". A key feature of the UL 910 test is the Steiner Tunnel test (horizontal forced air draft) as modified for communications cables. During the UL 910 test, flame spread values are observed for a predetermined time (20 minutes under the current standard), and smoke is measured by a photocell in an exhaust duct. Data from the photocell measurements are used to calculate peak and average optical density values. Specifically, according to UL 910, the measured flame spread must not exceed five feet, peak smoke (optical) density must not exceed 0.5, and average smoke (optical) density must not exceed 0.15. In general, for UL 1666, the measured flame spread must not exceed 12 ft. or 850.degree. F.
In order to meet the foregoing standards, various cable materials used in riser or plenum cables for the prevention, inhibition, and/or extinguishment of flame, may fall into two general categories. The first category includes inherently non-flammable, flame-resistant materials that are thermally stable, and may have high decomposition temperatures, for example, certain metals or high temperature rated plastics. The materials included in this first category can be useful as thermal/heat/flame barriers. Thermal/heat/flame barriers may have disadvantages, however, as they can be generally expensive and, because of limited burn-performance characteristics, they may be limited to a narrow range of applications.
The second general category of materials used for the prevention, inhibition, and/or extinguishment of flame includes inherently flammable materials that have been chemically altered to include flame retardant additives. Such additives actively interfere with the chemical reactions associated with combustion. Examples of inherently flammable materials are polyethylene, polypropylene, polystyrene, polyesters, polyurethanes, and epoxy resins. Typical flame retardant additives include aluminum trihydrate, metal hydroxides, brominated and chlorinated organic compounds, and phosphate compounds.
By comparison, thermal/heat/flame barriers typically do not include flame retardant additives, but rather are relied upon in flame protection designs for their resistance to decomposition at high temperatures, or their inherent heat dissipation properties. An example of a fiber optic cable that requires a thermal barrier, and is designed for use in plenum applications, is disclosed in U.S. Pat. No. 4941729, and is incorporated by reference herein.
Exemplary known composite cables may not meet flame, water penetration, mechanical, and/or environmental cable performance standards, and may not be suitable for all indoor or indoor/outdoor applications. For example, U.S. Pat. No. 5544270 discloses a composite cable having multiple twisted pairs of electrical conductors in combination with optical fiber conductors. The cable has interstices between the twisted pair conductors and optical fiber conductors and, as none of the interstices include water blocking components, the cable may not meet water penetration standards.
Another composite cable that may not be suitable for all indoor or indoor/outdoor applications is disclosed in U.S. Pat. No. 5539851. The cable includes a single central, tight buffered optical fiber surrounded by a ring of electrical conductors and a braided sheath RFI shield. The optical fiber is immediately surrounded by a KEVLAR sleeve and a TEFLON jacket. Because the composite cable has a single fiber, it has limited information carrying capacity. Additionally, the composite cable does not provide water blocking features in the interstices adjacent to the electrical conductors. Moreover, the combination of a KEVLAR sleeve, TEFLON jacket, a ring of electrical conductors, and a braided sheath results in a large, stiff composite cable that is not particularly suited to being routed through cable passageways.
A cable that may be suitable for use in indoor applications is disclosed in U.S. Pat. No. 5481635. The cable includes a single large, central broadband coaxial cable, a set of voice-line twisted pair conductors, and a set of power conductors disposed around the coaxial conductor. Water blocking members are disposed about the coaxial cable. Compared to a fiber optic core, however, a coaxial core is disadvantageous because it has a smaller bandwidth capacity, and is subject to higher power loss. Moreover, the coaxial conductor is subject to electromagnetic interference, impedance, and electrical cross talk. Further, the coaxial conductor core is generally relatively heavier and larger, rendering it potentially difficult to route through cable passageways. Additionally, the coaxial conductor presents a spark hazard. Finally, because the coaxial conductor emits electromagnetic energy, it is easier to tap and is therefore less secure than a optical fiber core.